A lot will depend on the attitude and ability of the club inspector if you go down the club route.
Non destructive testing of your welds should only cost £100, Club inspector may accept that.
Independant boiler inspector if you don't want the club to do it about £200
Insurance from a specialist if your club won't cover you will not be too bad from someone like Walker Midgely.
Hopefully Fizzy will be along later, have a look for his post "steel boiler for £200" to see it can be done at very reasonable cost if your welding is up to the job and you have an agreeable club inspector.